The present invention relates to an electrochemical system. More specifically, the present invention relates to electrochemical hydrogen flow rate control system.
Fuel cells that employ hydrogen gas as a fuel have known and used industrial applications. Such fuel cells are expected to be more developed in future as power sources capable of generating energies with less environmental load or impact. From a technical view for widespread use of fuel cells, one concern is how to effectively control hydrogen gas.
It is anticipated that hydrogen gas will be widely spread to societies in the future, and more specifically, hydrogen gas will be directly distributed to home, and thus readily and directly available at home. In general, floating type flow meters are now often used to control the flow rate of hydrogen gas. Such a floating type flow meter, however, is disadvantageous in causing a large error. Accordingly, it becomes important to develop a more accurate hydrogen flow rate control system.
On the other hand, a method of electrically controlling the flow rate of hydrogen gas has been adopted, wherein an error of a floating type flow rate control system is electrically detected, and the flow rate of hydrogen gas is controlled while the detected error was finely adjusted. Such a method, however, has a problem that the control system becomes too large and complex, and therefore, from the viewpoint of personal use, that is, to allow a user to readily control the flow rate of hydrogen gas, the control system is undesirable.
To solve the above-described problem, there has been proposed a method of converting hydrogen gas into protons by making use of a proton conductor, thereby controlling an amount of hydrogen gas as the amount of a current. With this method, it is possible to more accurately control the flow rate of hydrogen gas.
The conventional proton conductor is represented by a proton (hydrogen positive ion) exchange membrane mainly made from polytetrafluoroethylene or the like, which is operable at a temperature near room temperature. For example, a solid polymer proton conductive membrane is commercially available from Du Pont in the trade name of NAFION.
FIG. 6 is an enlarged sectional view of a related art hydrogen gas control system using a solid polymer proton conductive membrane as a proton conductor. As shown in FIG. 6, reference numeral 51 denotes a solid polymer proton conductive membrane, for example, NAFION having a thickness of about 0.2 mm, 52 is a gas diffusive anode electrode on which a catalyst such as platinum is supported, 53 is a gas diffusive cathode electrode on which the same catalyst as that for the anode electrode 52 is supported, 54 is a gas flow passage on the anode 52 side, 55 is a gas flow passage on the cathode 53 side, 56 is a metal current collector on the anode 52 side, and 57 is a metal current collector on the cathode 53 side.
The operational principle of this system will be hereinafter described. When hydrogen gas is supplied to the gas flow passage 54 on the gas diffusive electrode 52 side, it loses electrons on the gas diffusive electrode 52, to generate H3O+ ions in accordance with the reaction expressed by the following formula (1):H2+2H2O→2e−+2H3O+  (1)
The H3O+ ions thus generated migrate, together with moisture in the solid polymer proton conductive membrane 51, to the other gas diffusive electrode 53 by a drive force given by a voltage, and receive electrons on the gas diffusive electrode 53, to be converted again to hydrogen gas in accordance with the reaction expressed by the following formula (2):2e−+2H3O+→H2+2H2O  (2)
The hydrogen gas generated on the gas diffusive electrode 53 does not pass through the solid polymer proton conductive membrane 51. The migration force of ions given by the voltage is large. The metal current collectors 56 and 57 function to apply a voltage between the gas diffusive electrodes 52 and 53, and also function to mechanically reinforce the gas diffusive electrodes 52 and 53 and the solid polymer proton conductive membrane 51.
According to the solid polymer proton conductive membrane 51, for example, NAFION used for the related art electrochemical hydrogen gas flow rate control system as shown in FIG. 6, since the operating temperature at which a sufficient proton conductivity can be obtained is in a range of 80 to 100° C., there does not occur any inconvenience due to the operating temperature.
The solid polymer proton conductive membrane 51, however, has problems that a sufficient amount of moisture must be supplied to the membrane 51 to sustain effective proton conductivity, and that since moisture migrates along with migration of H3O+ ions in the membrane, it is required to supplement moisture even to the anode 52. As a result, the related art hydrogen gas flow rate control system must be provided with a large-sized humidifier, which leads to enlargement and complexity of the system.
In addition to the problem associated with the need of supplementing a large amount of moisture to the control system, the related art control system has a further problem that since moisture is generated on the cathode 53 side when hydrogen gas is generated on the cathode 53 side as shown in the formula (2), the hydrogen gas generated on the cathode 53 side contains a large amount of the moisture, with a result that it is difficult to control the amount of hydrogen gas to be generated.
A need, therefore, exists to provide improved electrochemical systems, particularly electrochemical systems that hydrogen-powered fuel cells.